1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing the same and, more particularly, a self-aligned stepped substrate (S3) type semiconductor laser and a method of manufacturing the same.
2. Description of the Prior Art
The structure of the S3 type semiconductor laser in the prior art is shown in FIG. 1.
In FIG. 1, an n-type cladding layer 102 formed of n-AlGaInP, a strained quantum well active layer 103, a first p-type cladding layer 104 formed of p-AlGaInP, a pn alternatively-doped (current) blocking layer 105 formed of AlGaInP, a second p-type cladding layer 106 formed of p-AlGaInP, and a contact layer 107 formed of p-GaAs are formed in sequence on an n-GaAs substrate 101 on which a step 101a having a inclined plane is formed. Respective layers 102 to 107 on the n-GaAs substrate 101 have a inclined plane that is almost parallel with the inclined plane of the step 101a respectively. Also, the alternatively-doped blocking layer 105 is formed due to the property that the n-type impurity is incorporated readily into the flat portion when the p-type impurity and the n-type impurity are supplied alternatively in growing. In contrast, the p-type impurity is incorporated preferentially into the portion that is parallel with the inclined plane of the step 101a of the AlGaInP layer constituting the alternatively-doped blocking layer 105 to thus form the p-type cladding layer 104.
In FIG. 1, the n-type cladding layer 102 and the p-type cladding layers 104, 106 are divided into first to fourth layer regions 111 to 114 by broken lines. The first to fourth layer regions 111 to 114 are portions in which a ratio of the flow rate of the group V material gas to the flow rate of the group III material gas (referred to as a xe2x80x9cV/III ratioxe2x80x9d hereinafter) is changed respectively or portions in which the growth temperature is changed respectively.
More particularly, the first and fourth layer regions 111, 114 are the portions that are formed by the high V/III ratio or at the low growth temperature, and the second and third layer regions 112, 113 are the portions that are formed by the low V/III ratio or at the high growth temperature. Explanation will be made hereunder by taking the steps of changing the V/III ratio of the material gas as an example, but the similar advantages and structure can be achieved by making a change of the growth temperature.
To differentiate the V/III ratio acts to change lines that define the flat portions and the step portions of the cladding layers 102, 104, 106, i.e., respective profiles of boundary lines between the flat surfaces and the inclined planes of the first to fourth layer regions 111 to 114 (referred to as xe2x80x9cgrowth profilesxe2x80x9d hereinafter). In FIG. 1, dot-dash lines denote growth profile lines indicating the change of the growth profile.
By the way, in the cladding layers 102, 104, 106, an angle xcex8 between the flat portion and the growth profile line is small in the portions which are grown by the high V/III ratio, and this angle xcex8 exhibits a tendency to increase as the V/III ratio is lowered. For example, an angle xcex801 of the growth profile line of the first layer region 111 is smaller than an angle xcex802 of the growth profile line of the second layer region 112, and an angle xcex803 of the growth profile line of the third layer region 113 is larger than an angle xcex804 of the growth profile line of the fourth layer region 114.
It has been found that, in the first to fourth layer regions 111 to 114, the angle xcex8 between the flat portion and the growth profile line affects the polarization plane of the laser beam in the laser oscillation and that the polarization plane becomes substantially perpendicular to the growth line. In the relationship between the beam shape of the laser beam and the direction of the polarization plane, based on the request to maintain the compatibility with the lasers having other structures, it is requested that the polarization plane should be set in the parallel direction with the inclined plane of the active layer 103. That is, the angle xcex8 must be set to about 90 degree to the inclined plane of the active layer 103.
In the structure in the prior art, in the cladding layers 102, 104, 106, the portions that have large influence on the polarization plane and are close to the active layer 103 are grown at the low V/III ratio to form their growth profile lines substantially perpendicularly to the inclined plane of the active layer 103, and also the polarization planes are set in parallel with the inclined plane portion of the active layer 103 by growing the portions that have small influence on the polarization plane and are far from the active layer 103 at the high V/III ratio. The inclined plane (step portion) of the active layer 103 is referred to as a stripe portion hereinafter.
The reason for that all the cladding layers 102, 104, 106 are not grown at the low V/III ratio is that, if the layer growth is carried out at the boundary portion between the n-GaAs substrate 101 and the GaAs contact layer 107 at the low V/III ratio, the crystal defect is ready to generate at the boundary portion between them and therefore such defect should be prevented.
Meanwhile, with the higher speed of the optical disk as the laser beam irradiation object, the optical output required for the semiconductor laser is increased year by year. As one factor to limit the higher output of the semiconductor laser, there is the kink in the current-optical output characteristic of the semiconductor laser.
As one factor to generate the kink, there is the event that normally the growth profile lines are not perfectly parallel with each other at the right and left portions of the stripe portion. If the components that modify the polarization plane differently are present at the right and left portions of the stripe portion, the transverse mode of the laser becomes unstable.
As the method of stabilizing such transverse mode, in Patent Application Publication (KOKAI) Hei 11-26884, it is disclosed that the transition region which appears when the low V/III ratio cladding layer is grown on the high V/III ratio cladding layer should be employed. The transition region has such a property that causes the right and left growth lines of the stripe portion to be formed in parallel to improve the kink level.
In FIG. 1, both the second and third layer regions 112, 113 are grown at the low V/III ratio. In this case, the second layer region 112 corresponds to the transition region that causes the growth profile lines to be formed in parallel, and the third layer region 113 corresponds to the stable region that appears after the transition region is completed. The active layer 103 is formed in the second layer region 112 serving as the transition region.
In order to get the characteristic in the 100 mW class by improving further the kink level, the further optimization of the layer structure is needed.
Major approaches are to narrow the stripe portion further and to strengthen the symmetrization of the growth profiles on both sides of the active layer 103.
FIG. 2 shows a schematic sectional view obtained when the narrower stripe formation and the growth profile symmetrization strengthening of the active layer are carried out by employing the technology in the prior art. In FIG. 2, in order to strengthen the symmetrization of the active layer growth profiles, the active layer 103 is provided in the center portion of the transition region that is grown at the low V/III ratio. Accordingly, two growth profile lines on both sides of the stripe portion of the active layer 103 become parallel.
However, in the structure shown in FIG. 2, following problems are caused in the device characteristics.
The first problem is that the polarization plane of the laser beam output from the semiconductor laser is ready to rotate. Normally, the low V/III ratio transition region 112 formed on the first layer region 111 serving as the high V/III ratio growth layer does not appear to have a thickness of about 0.5 xcexcm or more. Therefore, if this transition region is assigned to upper and lower portions of the active region 103, only the thickness of about 0.25 xcexcm can be given on one side. In contrast, in order to prevent the characteristic degradation due to the optical absorption of the GaAs substrate 101, the n-type cladding layer 102 needs the thickness of about 1.5 xcexcm at least and therefore an occupying rate of the high V/III ratio layer region 111 in the n-type cladding layer 102 becomes high. As the result, it is impossible to maintain the polarization plane of the laser beam in parallel with the stripe portion of the active layer 103.
The second problem is that a width between the right and left blocking layers 105 formed between the first p-type cladding layer 104 and the second p-type cladding layer 106 is narrowed because of the narrower stripe formation, and thus the device resistance is increased. The blocking layer 105 in the prior art is formed in the stabilized region that is grown at the low V/III ratio and serves as the third layer region 113. In this stabilized region, the growth profile lines are formed like the xe2x80x9cxcex9xe2x80x9d character as the layer thickness is increased, and thus there is the strong tendency that a width of the inclined surface of the p-type cladding layer 104 is reduced. As a result, the resistance increase of the p-type cladding layer 104 between the current blocking layers 105 formed on the upper end of the third layer region 113 is remarkable.
It is an object of the present invention to provide a semiconductor laser in which a layer structure capable of improving a kink level of an optical output characteristic is formed on a stepped substrate and a method of manufacturing the same.
According to the present invention, the first conduction type cladding layers which is formed under the active layer are formed on the stepped plane of the substrate to have the at least quadruple-layered structure, and the angle of the growth profile line of the inclined plane of the uppermost first conduction type cladding layer to the flat principal plane is enhanced by changing the angles of the growth profile lines of the inclined planes of the first conduction type cladding layers to the principal plane so as to repeat small, large, small, and large.
Accordingly, the angle of the growth profile line of the inclined plane of the first conduction type cladding layer that is formed just under the active layer can be set substantially perpendicular to the stripe-portion of the active layer. In addition, at least two layers that have the growth profile line at the angle that is substantially perpendicular to the stripe-portion of the active layer are provided alternately in the first conduction type cladding layers. Therefore, in the first conduction type cladding layers, a total layer thickness of the layers which have the growth profile line that do not become substantially perpendicular to the stripe-portion of the active layer can be reduced rather than the prior art. As a result, the rotation of the polarization plane of the laser beam is suppressed and thus the kink level of the output characteristic of the semiconductor laser is held high.
Also, according to the present invention, in the second conduction type cladding layers having the plural-layered structure formed over the active layer, the current blocking regions are formed on both sides of the inclined plane in the bottom region of the second conduction type cladding layer in which the angle of the growth profile line is small.
As a consequence, the inclination of the growth profile line of the second conduction type cladding layer that is formed along the inclined plane of the active layer and is put between the current blocking regions becomes small, but the growth profile of the region between the current blocking regions holds the almost same width even if the layer thickness is increased. Therefore, even if the stripe portion is formed as the narrow stripe, the second conduction type cladding layer between the current blocking regions is not narrowed and also the device electric resistance is never reduced. The reduction of the device electric resistance prevents the kink level of the output characteristic deteriorating.
In order to prevent the event that the width of the inclined plane of the second conduction type cladding layer formed between the current blocking regions becomes narrow, it is abandoned to grow the current blocking regions in the stable regions having the low V/III ratio, but the method of forming the current blocking regions at the high V/III ratio or the low growth temperature is employed.
In the layers that are grown at the high V/III ratio or the low growth temperature, the angles of the growth profile lines on the side portions of the inclined plane become small, nevertheless the tendency that the width of the stripe-portion is reduced as the layer thickness is increased is small in contrast to using the stable region growth that is carried out at the low V/III ratio or the high growth temperature. Therefore, the low V/III ratio growth is switched to the high V/III ratio growth in the thin thickness stage of the low V/III ratio stable growth, wherein the reduction in the width of the stripe-portion is not conspicuous, and then the increase of the device resistance can be prevented by forming the current blocking regions under these conditions by the pn alternative doping.